1. Field of the Invention
This invention relates to a motor vehicle door reinforcement tube having high strength, toughness and resilience for use as an impact absorption element for a motor vehicle door, in particular the door of a passenger car.
2. Background Information
These door reinforcement tubes are used as braces in motor vehicle doors to absorb the impact energy in lateral collisions and convert the impact energy into mechanical work, with the intent being to ensure the integrity of the door structure. For such a door reinforcement tube to perform this safety function, the tube must possess the following characteristics, for example:
______________________________________ Tensile strength RM min. 1100 N/mm2 Limit of elasticity Rt min 800 N/mm2 Elongation at failure A5 min. 8% ______________________________________
or a W-150 energy absorption of at least 1900 Joule, as a function of the profile dimensions.
These minimum requirements can be higher or lower, as specified by the automobile manufacturer.
The energy absorption is tested by deflecting a door reinforcement tube 150 mm in a 3-point bending test. The force applied is recorded over the deflection and the surface area below the curve is determined. This integral then gives the desired energy absorption characteristic.
In automobile construction, in addition to many special developments, two basic problems are constantly being addressed: the reduction of the vehicle weight, and an increase in the passive safety provided to the seated passengers.
One proposed solution for these problems is disclosed in German Patent No. 37 28 476, in which, through the use of a particular steel alloy, the tensile strength and limit of elasticity values of a door reinforcement tube are increased compared to the prior art. One disadvantage of this proposal is that the reinforcing effect is achieved only by the addition of relatively expensive elements such as nickel and molybdenum.
According to another proposal, German Patent No. 36 06 024, a protective element made of light alloy is used to reinforce the vehicle door at both ends of the vehicle door. This nearly square reinforcement element is not intended for narrow vehicle doors with a low insertion depth, and is designed with thick walls to compensate for low strength values, which means that the advantage of the lower specific weight of the light alloy is largely eliminated. Moreover, this reinforcement element is not very adaptable to different vehicle doors, as different doors may require different dimensions. Also, for each reinforcement element that is needed, a new extrusion die needs to be created, which extrusion die is expensive to make.